Does an online tool upload your file?
You need to merge two PDFs, so you search, pick the first site, and drop in a contract, a passport scan, or a bank statement. Thirty seconds later the result downloads. Where did the file go in between? The page rarely says, and both answers look exactly the same from the outside.
Two ways to build the same button
The first way sends your file to the site’s computer. It does the work there and sends a new file back. Your document now exists in two places, and the second one is out of your sight. A good service deletes it after an hour. You are trusting that it does.
The second way does the work inside the page you already opened. Your browser is a capable program. It can read a PDF, reorder its pages, and shrink a photo without help. The site sends the instructions once, and your file never travels. There is nothing on the other end to delete.
Neither is automatically bad. Some jobs, like turning a scan into editable Word text, often need a bigger machine than a phone. But for merging, splitting, rotating, resizing, or compressing, the work fits on your own device.
How to check
On a computer, open the browser’s developer tools and choose the Network tab. Run the job once with a test file. An upload shows up as a request roughly the size of your file, going out. A tool that works locally only fetches its own code, and nothing that size leaves the page.
On a phone, try airplane mode. Run the job once with the connection on, so the page has everything it needs. Then switch off Wi-Fi and data, drop in a different file, and run it again. If the result still appears, the work happened on your device. If the page spins or asks you to check your connection, the file was meant to go somewhere.
Pages often fetch extra code the first time you use a feature, which is why the first run happens online. A tool that fails only on that very first offline try is not proof of an upload. A tool that fails every time without a connection usually is.
When it matters
For a holiday photo, it may not matter much. For a medical letter, an ID scan, a payslip, or a client contract, the safest copy is the one that never left your laptop. Workplaces with data rules often care about exactly this question, even when the person dropping in the file does not think to ask.
Every SharpSloth page works the second way. Open the Network tab while you use one and you will see the page load its code, and no request carrying your file.